Oven protective device



United States Patent 3,281,567 OVEN PROTECTIVE DEVICE George W. Meissner, Fridley, and Roger L. Knutson, Lake City, Minm, assignors, by mesne assignments, to Litton Precision Products Inc, Beverly Hills, Calif, a corporation of Delaware Filed Oct. 23, 1963, Ser. No. 319,358 6 Claims. (Cl. 219-1055) This invention relates to electronic heating apparatus and, more particularly, to apparatus utilizing radiated high frequency energy for heating food.

High frequency heating apparatus such as is commonly used for electronic cooking comprises a metallic cabinet forming a substantially closed cooking chamber, and a microwave generator, for example, a magnetron oscillator device, discharging microwave energy into the cooking chamber. In general, a magnetron cannot operate under no-load or mismatch conditions for under these circumstances, a high standing wave ratio is developed causing the magnetron to overheat and fail within a very short period of time. It has been proposed that dissipative means be affixed to the cabinet to absorb part of the energy at no-load conditions and thereby prevent damage to the magnetron. This solution, suggested for example in Blass et a1. Patent No. 2,704,802, has only limited usefulness and it has been found more desirable to detect a no-load condition and positively terminate power to the magnetron in order to prevent damage thereto.

An object of the present invention is to provide high frequency electronic heating apparatus with novel noload detecting means.

Another object of this invention is to provide a high frequency electronic heating apparatus such as a microwave oven, with protective means for detecting a no-load condition therein and for terminating the supply of power to the microwave energy generator.

Yet another object of the present invention is to provide a microwave oven with a no-load detector comprising a box-like member defining a chamber or cavity which communicates with the interior of the microwave oven through aperture means defined in the wall of the microwave oven in a high microwave energy region, the boxlike member having sensing means therein for detecting a no-load condition and for actuating means for interrupting the power circuit to the microwave generator.

These and other objects and advantages of the present invention will become more apparent from a reading of the following specification in conjunction with the attached drawing, wherein:

FIGURE 1 is a side view of a high frequency electronic heating apparatus embodying the no-load detecting means of the present invention, with parts being broken away for clarity;

FIGURE 2 is a front view of the high frequency electronic heating apparatus embodying the present invention taken along line 22 of FIGURE 1 and schematically illustrating a portion of the electrical circuitry associated therewith; and

FIGURE 3 is a cross-sectional view on an enlarged scale of a portion of the no-load detecting means of the present invention taken generally along line 33 of FIG- URE 1 and more clearly illustrating the connection of the housing of the no-load detecting means to the cabinet of the high frequency electronic heating apparatus.

Referring now to the drawing and, more particularly, to FIGURES 1 and 2, there is illustrated a high frequency electronic heating apparatus for reconstituting or cooking food. Such apparatus, commonly referred to as a microwave oven, may comprise a rectangular prismoidal enclosure or cabinet 10 defining therewithin a cooking cavity ice or chamber 12. The cabinet 10 is preferably fabricated from a suitable metal and has rat-her thin walls as indicated in the drawing. The access opening to cavity 12 is closed by a door 11.

Provided in the bottom 13 of the enclosure means defining the cooking cavity 12 is a microwave oscillation generator 14, preferably a magnetron for radiating high frequency electromagnetic energy into the cooking chamber.

Suitably carried on support members, as for example, angle members 17 within the cavity 12 is a rack 18 upon which the food 20 to be reconstituted or cooked may be supported.

The dimensions of the cavity 12 are ordinarily large with respect to the wave lengths of the radio frequency energy generated from magnetron 14 to render the heating operation within the microwave oven more uniform. It has been found desirable to periodically or cyclically change the effective internal dimensions of the cavity 12. Accordingly, rotatable stirrer means 22 may be provided in the upper portion of the cavity to periodically change the radio frequency field pattern. As the field distribution or the mode pattern inside the cavity is periodically changed by means of the motion of the stirrer means 22, which may be driven by a suitable drive mechanism 24, there is provided within the oven a more uniform heating for cooking purposes.

When food is present within the microwave oven the high frequency energy emitted from the magnetron 14 will be absorbed by the food. However, in the event that there is no food within the cavity 12, there is a danger that a high standing wave ratio will be developed, ultimately causing damage to the magnetron. Those familiar in the art of microwave oven cooking have observed that under very light load or no-load conditions the electromagnetic field concentration greatly increases on the oven enclosure surfaces. This phenomenon is particularly noticeable around openings in the microwave oven enclosure, such as the access opening. The present invention advantageously utilizes this characteristic in a novel manner for providing a microwave oven with means for detecting a no-load condition within the microwave oven and for terminating the supply of power to the microwave oscillation generator.

No-load detecting means 26 have been provided to sense a no-load condition and to positively terminate the power supply to the magnetron. The no-load detecting means 26 comprises a housing 23 which is affixed to a wall 27 of the cabinet 10 by suitable fastening means 29 which may comprise, for example, screw members, and sensing means associated therewith. The housing or enclosure 28 defines a cavity 30 which is in communication with the cooking cavity 12 by means of an aperture 32 provided in the wall 27 of the cabinet 10. The aperture 32 may be uncovered, or if desired, it may be covered with a material such as plastic or glass, which does not interfere with the passage of microwaves. The cavity 3%) is preferably designed as a resonant chamber with the walls 33 spaced apart at approximately /2 Wave lengths.

The aperture typically is approximately /2 wave length in lenghh and the width thereof is dependent on the desired power input to cavity 30. The aperture 32 is defined in the wall 27 in an area of high microwave energy and is adapted to couple the chamber :12 and the cavity 30 when there is no food in chamber 12. The aperture 32 is located in a side wall of the cabinet 10 in a high energy area, for it has been found that the device will not perform satisfactorily if the slot is in a corner of the cabinet or is at the top thereof above the stirrer means 22.

In one presently preferred form of the invention, the aperture 32 is defined as a slot approximately 2% inches long by A inch wide. The slot is preferably formed in.

the wall member 27 at an angle of 45 degrees. In this construction, the oven cavity 1-2 is 9 inches high by 12 inches wide by 14% inches deep and the cavity 30 defined within housing 30 is approximately 2.4 inches along each side. It will be understood that these dimensions apply to one presently preferred form of the invention and that the invention can be applied to ovens of dilierent sizes and configurations.

'Ihe sensing means or sensor assembly 34 may be afiixed to the top of the housing 28 and extend thereinto through an opening 35 in the top wall member 33. The sensor assembly 34 comprises a generally cylindrical metal shell 36 which serves as a mounting for a photocell 37. Preferably, the shell 36 is made from brass and is rotatably supported on housing 28. Operatively connected to the photocell 37 is a light bulb 38. As will be apparent to those versed in the art, diflerent types of photocells can be employed. If a photocell is used which produces only a very small current, then an amplifier 39 may be required in the electrical circuit to the relay or like switch mechanism 40 which is adapted to open the power circuit 41 to magnetron 14. On the other hand, if a photocell is used which produces a relatively large current, then an amplifier will not be needed in the electrical circuit to relay mechanism 40.

Referring now to FIGURE 3, it is seen that the light bulb 38 is of a type sometimes referred to as a peanut bulb. The bulb 38 has a pair of leads or antennas 44 extending therefrom into the cavity 30. The leads 44 are pre-formed in a manner so as to act as an antenna for stray radio frequency. Thus, the relative angular and lateral positioning of the leads 44 within the resonant cavity 30 can be used to absorb the correct predetermined amount of energy under no-load conditions. The leads 44 extending into cavity 30 from bulb 38 are preferably from 0.100 to 0.750 inch long. The length of the leads is selected in a manner to increase the sensitivity of the noload detecting means. If a relatively small quantum of energy is discharged into chamber 30, longer leads 44 are used. If a relatively large quantum of energy is provided in chamber 30, the leads 44 are shorter. The sensitivity of the sensor assembly may be adjusted by rotating the shell 36 in the opening 35. The leads may be retained in the predetermined position by encapsulating them in a low-loss epoxy material indicated generally at 46. An example of a satisfactory low microwave absorption epoxy is -R.T.V.-102, a room temperature vulcanizing epoxy made by the General Electric Company. It will be recognized that the leads may, if desired, be covered with glass, Pyrex or the like material which will pass high frequency waves. The adjustment of the position of the leads 44 is such that the filament 48 of the bulb will light or flash under no-load conditions or very light load conditions within the microwave oven cabinet but will not flash when the cooking cavity '12 is loaded in a predetermined manner.

Operation In operation, the resonant cavity 30 will not receive sufiicient energy from the oven cavity 12 to energize bulb 38 when there is a load therewithin. If there is no load within the cavity 12, energy will build up within the resonant cavity 30 and at a predetermined level, the filament 48 of the light bulb 38 will flash or light. The light emitting from bulb 38 is absorbed by the photocell 37. Current is produced in the circuit to relay 40. If needed, amplifier means 39 may be provided to amplify the current to the relay mechanism 40. The relay 40 will be actuated to open the power circuit 41, thus terminating the flow of power to the magnetron. In this manner, positive protection is afforded for the magnetron in the event the microwave oven is actl lated with no load in the cooking cavity 12,

By the present invention, there has been provided novel microwave oven protection means which are capable of detecting a no-load condition or a predetermined light load condition within the cooking oven and which are operative to actuate means for terminating the supply of power to the microwave generator, thus preventing damage thereto.

The above disclosure has been given by way of illustration and elucidation, and not by way of limitation, and it is desired to protect all embodiments of the herein disclosed inventive concept within the scope of the appended claims.

We claim:

1. A microwave oven comprising a cabinet defining a cooking chamber; means for radiating microwave energ into said cabinet; means for supporting an article to be heated in said chamber; and means for detecting a no-load condition within said cooking chamber comprising a housing defining a cavity operatively communicating with said cooking chamber by means of aperture means defined in a cabinet wall in a region of high microwave energy, and sensing means in said cavity actuated in response to a noload condition for terminating the supply of current to said radiating means, said sensing means comprising a light bulb having antenna-like leads extending into said cavity and being constructed and arranged to light in response to a predetermined energy level within said cavity and a photocell responsive to lighting of said light bulb for causing opening of a protective circuit and thereby terminating the supply of power to said radiating means.

2. High frequency electronic heating apparatus comprising, in combination, a metallic enclosure defining a cooking chamber, means for supplying high frequency electromagnetic energy to said cooking chamber for cooking food contained therein, stirrer means within said cooking chamber for producing a cyclically repetitive distribution of energy which is substantially uniform in the cookng region of the cooking chamber, means for detecting a no-load condition in the cooking chamber comprising a housing defining a cavity therein communicating with the cooking chamber through an aperture defined in a wall of the metallic enclosure in a region of high microwave energy, said aperture being about one-half wave length long, and sensing means in said cavity for sensing a predetermined energy level when no food is in the cooking chamber and terminating the supply of power to the electromagnetic energy supplying means.

3. High frequency electronic heating apparatus comprising, in combination, a metallic enclosure defining a cooking chamber, means for supplying high frequency electromagnetic energy to said cooking chamber for cocking food adapted to be supported therein, and means for detecting a no-load condition in the cooking chamber and terminating the supply of power to the means for supplying high energy electromagnetic energy, said detecting means comprising a housing defining a cavity therein communicating with the cooking chamber through aperture means defined in a wall of the metallic enclosure, sensing means in said cavity including a photocell for sensing a predetermined energy level when no food is in the cooking chamber, and means operable in response to actuation of said photocell for terminating the supply of power to the means for supplying high frequency electromagnetic energy.

4. A microwave oven comprising a metallic enclosure defining a cooking chamber, means for radiating microwave energy into said chamber, means supplying power to said microwave energy radiating means, means for supporting a body to be heated in said chamber, one wall of said enclosure being provided with an aperture therein in an area of high microwave energy, a housing having a cavity defined therein communicating with said chamber through said aperture, said aperture being operative to couple said chamber and said cavity when there is no body within said chamber and being inoperative to couple said chamber and said cavity when said body is disposed in said chamber, light generating means in said cavity actuated when said chamber and said cavity are coupled and there is at least predetermined energy level in said cavity, and means responsive to lighting of said light means to terminate the flow of power from said power supplying means to said radiating means.

5. A microwave oven comprising a metallic enclosure defining a cooking chamber, means for radiating microwave energy into said chamber, means supplying power to said microwave energy radiating means, means for supporting a body to be heated in said chamber, one wall of said enclosure being provided with an aperture therein in an area of high microwave energy, a housing having a cavity defined therein communicating with said chamber through said aperture, said aperture being operative to couple said chamber and said cavity when there is no body within said chamber and being inoperative to couple said chamber and said cavity when said body is disposed in said chamber, light generating means in said cavity actuated when said chamber and said cavity are coupled and there is at least a predetermined energy level in said cavity, and means responsive to lighting of said light means to terminate the flow of power from said power supplying means to said radiating means, said means responsive to lighting of said light generating means comprising a photocell, amplifying means for amplifying the control current supplied from the photocell when it is actuated upon lighting of the light generating means, and switch means operatively associated with said amplifying means for terminating the flow of power from said power supplying means to said radiating means.

6. A microwave oven comprising a metallic enclosure defining a cooking chamber, means for radiating microwave energy into said chamber, means supplying power to said microwave energy radiating means, means for supporting a body to be heated in said chamber, one wall of said enclosure being provided with an aperture therein in an area of high microwave energy, a housing having a cavity defined therein communicating with said chamber through said aperture, said aperture being operative to couple said chamber and said cavity when there is no body within said chamber and being inoperative to couple said chamber and said cavity when said body is disposed in said chamber, light generating means in said cavity actuated when said chamber and said cavity are coupled and there is at least a predetermined energy level in said cavity, and means responsive to lighting of said light means to terminate the flow of power from said power supplying means to said radiating means, said means responsive to lighting of said light generating means comprising a photocell actuated upon lighting of the light generating means, and switch means operatively associated with said photocell for terminating the flow of power from said power supplying means to said radiating means.

References Cited by the Examiner UNITED STATES PATENTS 2,404,147 7/1946 Stricklands 219-1077 X 2,595,748 5/1952 Andrews 21910.55 2,647,983 8/1953 Boyd 219-1077 2,929,905 3/1960 Hahn 219-1055 3,182,166 5/1965 Bohm et al 2l9--10.55 3,185,809 5/1965 Bohrn et a1 219l0.55

RICHARD M. WOOD, Primary Examiner.

L. H. BENDER, Assistant Examiner. 

1. A MICROWAVE OVEN COMPRISING A CABINET DEFINING A COOKING CHAMBER; MEANS FOR RADIATING MICROWAVE ENERGY INTO SAID CABINET; MEANS FOR SUPPORTING AN ARTICLE TO BE HEATED IN SAID CHAMBER; AND MEANS FOR DETECTING A NO-LOAD CONDITION WITHIN SAID COOKING CHAMBER COMPRISING A HOUSING DEFINING A CAVITY OPERATIVELY COMMUNICATING WITH SAID COOKING CHAMBER BY MEANS FOR APERTURE MEANS DEFINED IN A CABINET WALL IN A REGION OF HIGH MICROWAVE ENERGY, AND SENSING MEANS IN SAID CAVITY ACTUATED IN RESPONSE TO A NOLOAD CONDITION FOR TERMINATING THE SUPPLY OF CURRENT TO SAID RADIATING MEANS, SAID SENSING MEANS COMPRISING A LIGHT BULB HAVING ANTENNA-LIKE LEADS EXTENDING INTO SAID CAVITY AND BEING CONSTRUCTED AND ARRANGED TO LIGHT IN RESPONSE TO A PREDETERMINED ENERGY LEVEL WITHIN SAID CAVITY AND A PHOTOCELL RESPONSIVE TO LIGHTING OF SAID LIGHT BULB FOR CAUSING OPENING OF A PROTECTIVE CIRCUIT AND THEREBY TERMINATING THE SUPPLY OF POWER TO SAID RADIATING MEANS. 